This invention relates to optical devices known as quarterwave retarders. In particular this invention relates to an achromatic quarterwave retarder prism for which the emerging light beam is on the same axis or collinear with the entering light beam. The device according to the present invention is achromatic and may also be referred to as a broadband device in that it will work as intended for a wide range of light wavelengths about a preselected wavelength. The device according to the present invention may have general optical applications, but it is found to be particularly useful in the optical recording art.
The optical recording art in a typical application has the laser light beam passing through an optical system including a quarterwave device to a moving optical medium from which the light modulated by information from the moving media is reflected back through the same quarterwave device and reflected in a polarizing beam splitter in a different direction to a light sensitive device which reads the intensity of the reflected light beam from the optical media. Use of quarterwave devices and the optical systems employing them in the optical recording art is discussed in numerous U.S. patents. Of particular interest is U.S. Pat. No. 3,969,573 which discusses the requirements of the quarterwave retarder as used in optical recording in great detail as well as showing as an invention a different type of quarterwave device which is unrelated to the present invention. However, that patent does illustrate that the need for a suitable quarterwave device is critical to the optical recording art. A typical quarterwave retarder used in the optical recording art is a so-called quarterwave plate having end faces parallel to the optical axis.
Other types of quarterwave retarders are known in the optical field. One useful device is the Fresnel-rhomb which is described along with other similar devices by J. M. Bennett in an article entitled "A Critical Evaluation of Rhomb-type Quarterwave Retarders," Applied Optics, Volume 9, No. 9, pgs. 2123 through 2129, September 1970. Although the Fresnel-rhomb is well known in optics, its use in optical recording would be difficult because the two internal reflections which occur in a Fresnel-rhomb cause the emerging light beam to be displaced with respect to the entering light beam. This displacement in the light path causes complications in optical system implementation. The dimensions are already critical in optical recording systems. Therefore, it is desirable in optical recording systems to have an on-axis or collinear quarterwave retarder device.
Also known in the prior art is U.S. Pat. No. 3,635,563 which shows a parallelogram-shaped prism in which the light undergoes four total internal reflections and emerges on a different optical axis than the entering optical axis. The entering and emerging light beams pass through prism faces which are at right angles to the optical axis. However, this patent does not anticipate the present invention although it does illustrate the use of the laws of optics as applied to internal reflections in prisms.